In pneumatic tires, the cord reinforcements used as the carcass and the cap ply has twisted structures and circular cross-sections (FIG. 1). The twist directions of yarns and cords are expressed with letters S and Z. When the twisted yarn or cord is in vertical position, if the spiral bendings around the central axis is parallel to the middle portion of the letter S, it is defined as S twisted (S-like twist); if it is parallel to the middle portion of the letter Z, it is defined as Z twisted (Z-like twist).
In a rubberized (calendered) tire cord fabric, there are additional rubber layers on the top and bottom sides of the cords in order to provide required tire performance. On the other hand, rubber matrix causes heat buildup and rolling resistance under dynamic conditions because of its viscoelastic behavior.
The ply thickness is calculated according to a cord diameter having required breaking strength for a given ply strength and optimized number of cords per unit length (epdm). In order to decrease the rubber and tire weight, the ply thickness should be decreased. With this purpose, the cord diameter can be reduced by increasing the epdm (cord count) without changing the unit ply strength. But the increase in epdm means also a reduction in spacing between the cords which may cause poor bending and compression fatigue resistance in tire. For this reason, it is a very limited possibility to increase epdm without sacrificing tire durability.
According to U.S. Pat. No. 6,959,534 of Goodyear, in order to reduce twisting cost, the tire cords having lower ply twists than that of cable twists have already been used in tire industry as reinforcement. According to the said patent, the dipped/heat-set and calendered cords are difficult to process due to their residual torques, caused by the unbalanced ply and cable twists. The residual torques result in the tip rise and curlings in the dipped, and rubberized cord fabrics. For this reason, the cords having S and Z twists are placed alternately in the fabric, in the said Goodyear patent application, instead of conventional tire cord fabric made of fully S twisted cords.
In the application, it is stated, when the cords having unbalanced S and Z twists used alternately in the cord fabric, the torques formed in dipped and calendered cord fabric in S and Z direction would neutralize each other and eliminate the process problems. However, the said patent does not mention the potential superiority of mutual Z and S twisted cords with respect to reduced shear strain between them. Besides the technical advantage which was not mentioned, the unbalanced twisted cords disclosed in the patent have lower breaking strength and modulus compared to the corresponding balanced twisted cords.
The Z and S twisted alternate cord applications can also be seen in some steel cord belts. The objective in the said applications is to eliminate the handling problems (curling) of the calendered steel cord layers.
Patent document numbered GB243705 present in the state of the art related with tire reinforcement fabric having side by side cords (warps) having opposite twist directions. The possible goal could be balancing residual cord torques which may result in tip rise or curling in fabric during processing. Nevertheless, it does not mention the potential advantage of side by side cords with opposite twisting direction in tire performance, especially bending and compression fatigue resistance. The other weak point of this reference document is the lack of the spacing (rivet) between the paired strips. Such fabrics without any rivet do not function in bias or radial tires and results in cord breaks under bending and compression.
Another patent document present in the state of the art EP1491365 has nothing to do with paired cords. In order to obtain a cord, 2 or 3 yarns are twisted in Z direction, and then those twisted yarns are twisted altogether in opposite direction (S direction). RFL treatment, which is mentioned in EP11491365, is already a well known process to obtain adhesion between cord and rubber. However, this document does not mention any alternating twisted side by side cords. Similarly, patent document numbered EP1731333 proposes the same technical solution to the same technical problem; but it does not mention the paired cords.
FR568042, which is another patent document in the state of the art, is related with tire reinforcement fabric having side by side cords (warps) having opposite twist directions. In the specification of this document, the explained goal is balancing residual cord torques. Currently, the textile cords used in tire reinforcement do not show any residual torque influencing processing in a negative way.